The present invention relates to the magnetic resonance imaging art. It finds particular application in conjunction with insertable gradient coils for imaging the wrist and will be described with particular reference thereto. However, it is to be appreciated that the invention will also find application in conjunction with other types of insertable magnetic field gradient coils.
Magnetic resonance imagers commonly include a large diameter, whole body gradient coil which surrounds a patient receiving bore. Main field magnets, either superconducting or resistive, and radio frequency transmission/reception coils also surround the bore. Although the whole body gradient coils produce excellent linear magnetic field gradients, they have several drawbacks. With large diameter gradient coils, the slew rate is sufficiently slow that it is a limiting factor on the rate at which gradient magnetic fields can be induced and changed. Large diameter whole body gradient coils have relatively low gradient field per unit Ampere for given inductance which limits their use for some of the highest speed magnetic resonance imaging techniques. The energy stored in gradient coils is generally proportional to more than the fifth power of the radius. Hence, large diameter, whole body coils require much larger amounts of energy. Further, superconducting main magnets have cold shields disposed around the bore. The larger the diameter of the gradient coil, the closer it is to the cold shields and, hence, the more apt it is to produce eddy currents. More shielding is needed to prevent the whole body gradient coils from inducing eddy currents in the cold shields than would be necessary for smaller diameter coils.
Due to these and other limitations in whole body coils, numerous insertable coils have been developed which are small enough to fit within the bore with the patient. Typically, the insertable coils are customized to a specific region of the body, such as a wrist coil. Traditionally, wrist coils have been a cylinder sized to accommodate the human hand, wrist, and forearm, e.g. 14-17 cm in diameter.
One of the difficulties with such insertable cylindrical gradient coils is that they need to be positioned at or near the isocenter of the magnet with their central axis aligned with the primary magnetic field.
In conventional whole body magnetic resonance imagers, the main magnetic field aligns axially with a central bore of a superconducting magnet assembly. The central bore is typically about 60 cm in diameter, and about 2 m in length. Part of the bore is lost to the patient supporting couch and other associated equipment. Moreover, it is advantageous for the patient to be generally centered in the bore. If the patient is pressed against the bore, the patient may be so close to the surrounding gradient and RF coils that artifacts are induced in the resultant image. With these constraints, it is very difficult to position a patient comfortably in the bore with his/her wrist at the isocenter and with the central axes of the wrist coil and the main magnet bore aligned.
Although the patient's forearm can physically be positioned along the longitudinal centerline of the chest, such a position places the wrist over the heart. Placing an insertable coil so close to the heart is generally considered a safety risk. Accordingly, less comfortable, but safer, positioning of the patient is commonly selected. Commonly, positioning the patient face down on the patient couch with the arm in question extended overhead is considered the most comfortable patient position.
Greater patient comfort might also be attained by positioning the wrist toward one end of the bore. However, because the homogeneity of the main magnetic field is greatest near the isocenter and is significantly reduced towards the ends, positioning the patient's wrist toward the end of the bore results in reduced image quality and enhanced T.sub.2 * and phase effects.
Some of the prior art insertable coils which are suitable for the wrist are non-shielded coils. Because the insertable wrist coils are placed close to the main magnet shield, eddy current effects are present in non-shielded coils. Such eddy currents significantly increase the rise times and worsen the slew rates of the insertable gradient coil. Moreover, these deleterious effects occur asymmetrically and are non-linear over the imaging region.
In accordance with the present invention, a new and improved insertable gradient coil is provided which overcomes the above-referenced problems and others.